Fix docs cross reference in DeviceBuffer.prefetch

[bdice]

Description

Closes #1635.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

[leofang]

Hmmm... I wonder what's wrong

docstring of rmm._lib.device_buffer.DeviceBuffer.prefetch:10: ERROR: Unknown interpreted text role "py".

https://github.com/rapidsai/rmm/actions/runs/10199058213/job/28215990143?pr=1636#step:9:1264

[bdice]

This works now in the docs. I'll merge.

[bdice]

/merge

[jakirkham]

Thanks all! 🙏